1. Field of the Invention
This invention relates to a brake control system capable of changing a target slippage ratio to another so that the optimum braking force can be obtained upon braking of a vehicle.
2. Description of the Related Art
There is known a brake control system for motor vehicles, such as motorcars and motorcycles, which calculates slippage ratios for wheels with respect to a road surface based on the running speed of the vehicle and the peripheral speed of each wheel, and applies the optimum braking force to the vehicle based on the calculated slippage ratios.
In the brake control system, the hydraulic pressure of a brake is varied proportionally to the amount of depression of a brake pedal, thereby to increase a braking force until a predetermined slippage ratio (hereinafter called a "target slippage ratio"), at which the braking force becomes the maximum, is reached (hereinafter called "normal control mode"). Upon attainment of the target slippage ratio, the control mode is shifted to limit an increase in the hydraulic pressure of the brake so as to avoid a further increase in the slippage ratio (hereinafter called a "limiting control mode").
There is a relationship between the slippage ratio of each wheel with respect to the road surface and the coefficient of friction of the wheel and the road surface. Namely, the slippage ratio, at which a vehicle can stably be controlled, differs between road surfaces having a high coefficient of friction (hereinafter called a "high .mu. road") and others having a low coefficient of friction (hereinafter called a "low .mu. road").
A high .mu. road is usually assumed, in the conventional brake control system, in setting a target slippage ratio. Upon entering the limiting control mode, the coefficient of friction .mu. between the wheel and an actual road surface is then calculated to change the target slippage ratio to another, thereby to enlarge the range in which the normal control mode is applicable. There has been disclosed in Japanese Patent Application Laid-Open Publication No. 62-194963 such a technical idea of detecting the coefficient of friction .mu. between the wheel and the actual road surface at the time of shifting the control modes from normal to limiting.
However, in the conventional brake control system referred to above, the detection of the coefficient of friction .mu. is initiated after the normal control mode has been shifted to the limiting control mode. Thus, there was a danger of setting a suitable target slippage ratio too late, i.e., after the control mode shift.
There is the situation in which the coefficient of friction .mu. between the surface of an actual running road and each wheel changes with a variation in the condition of the road surface. For example, slippage ratios vary between a low .mu. road whose surface is Eisbahn, i.e., a frozen snow surface or the like, and a high .mu. road which has an asphalt surface or the like. Thus, when a target slippage ratio with respect to a so-called .mu. jump road surface (where the surface of a road changes from the low .mu. road to the high .mu. road, for example), is changed to another, and a normal increase in the hydraulic pressure of the brake is made under a conventional brake control mode, the braking force greatly varies due to time delays or the like, so that the feeling of braking of the vehicle deteriorates due to the resulting vibrations.
Further, when an unevenness or irregularity appears on the road surface while the motorcar or motorcycle is running, the vehicle greatly vibrates. Thus, when the velocity of the vehicle and the coefficient of friction of the road surface are determined based on the output of a sensor or the like, noise is excessively included therein. As a result, there is the possibility that a target slippage ratio cannot be obtained with predetermined accuracy.
If the target slippage ratio is set up in a state in which information other than the coefficient of friction of the road surface (such as the behavior of the vehicle, the condition of the road surface, etc.) has also been taken into consideration, then a further accurate control of braking of the vehicle can be carried out. Thus, there is a demand for a brake control system for setting a target slippage ratio based on information such as the behavior of the vehicle, the condition of the road surface, etc.
In this type of brake control system, a slippage ratio used for the control of braking of the vehicle is calculated from the peripheral velocity of each wheel and the velocity of the vehicle. Alternatively, even in the case of a drive power control apparatus for adjusting the ignition timing of an engine according to a rapid start and a variation in the coefficient of friction of the road surface thereby to control a driving force, the peripheral velocity of each wheel and the velocity of the vehicle are used to control the braking of the vehicle in the same manner.
The peripheral velocity of the wheel can be obtained by directly detecting the rotational speed of each wheel with the sensor. It is however difficult to directly detect the velocity of the vehicle with a sensor. In a vehicle such as a motorcycle in particular, whose weight and size are greatly restricted, the direct detection of the vehicle velocity is actually impossible. Thus, a method of estimating the velocity of the vehicle based on the peripheral velocity of each wheel or the acceleration of the vehicle is normally used.
Described specifically, for example, there are known methods: one for selecting a peripheral velocity of one of a plurality of wheels, which is rotated at the fastest speed, thereby to calculate an estimated velocity of a vehicle (see Japanese Patent Application Laid-Open Publication No. 63-78867) (hereinafter called a "Hi-select system); another one for integrating an acceleration or deceleration of a vehicle detected by a vehicle acceleration detector thereby to obtain an estimated velocity of the vehicle (see Japanese Patent Application Laid-Open Publication No. 63-24166); and a further one for comparing a peripheral velocity of each wheel and the present estimated velocity of a vehicle and updating the estimated velocity of the vehicle at a given acceleration or deceleration based on the result of comparison.
However, when each wheel slips on the road surface, there is often a situation in which the estimated velocity of the vehicle based on the peripheral velocity of the wheel is set to a larger value as compared with an actual velocity of the vehicle. Thus, when each wheel slips on the road surface upon starting the vehicle, for example, the first method, i.e., the Hi-select system, tends to cause improper control in the brake control system or the drive power control apparatus. In the second method referred to above, there is the possibility that noise is included in the acceleration or deceleration of the vehicle due to the vibrations of the vehicle, which occur according to the state of the road surface. Thus, when the acceleration or deceleration of the vehicle is integrated and updated, the noise is also integrated. Consequently, there is the possibility that the estimated velocity of the vehicle does not come close to the actual velocity of the vehicle. Further, the third method causes inconvenience in that the set predetermined acceleration or deceleration should be changed over to another in the case of travelling over two road surfaces, one having a low coefficient of friction (low .mu. road) and the other having a high coefficient of friction (high .mu. road).
Further, when such a braking control is effected on the motorcycle, the control of braking can be better carried out by inclining the vehicle from the vertical in a direction perpendicular to the running direction of the vehicle (hereinafter called a "banking angle"). As methods of detecting the banking angle, there have been proposed a method of detecting the distance between a predetermined portion of a vehicle and the road surface with a sensor (see Japanese Patent Publication Laid-Open No. 2-216355), a method of detecting a banking angle with an inclinometer, a method of determining a banking angle by computation based on a steering angle and an estimated velocity of a vehicle.
However, the inclinometer is very expensive. And in the method of measuring the distance between the road surface and the predetermined portion of the vehicle with the sensor and determining the banking angle based on the result of measurement, much noise is produced due to the reflection of light onto the road surface or an irregularity of the road surface, for example. It is thus difficult to determine the banking angle with high accuracy.
Further, the method of determining the banking angle by computation based on the steering angle and the estimated velocity of the vehicle tends to cause a poor or rough correlation between the steering angle and the banking angle while the vehicle is running at a high speed, resulting in reduced reliability.